An outboard engine unit of the above type is well known in the art (see, for example, JP-A-2010-221754). The outboard engine unit includes a transmission interconnecting a crankshaft of an engine and a propeller and having an output shaft carrying a water pump thereon. The outboard engine unit also includes an engine-cooling system and a transmission-cooling system disposed separately from the engine-cooling system. The water pump is driven by the output shaft of the transmission to draw water from an outside of the outboard engine unit and then pump out the water to the engine for cooling the engine. The engine is cooled by the engine-cooling system and the transmission is cooled by the transmission-cooling system.
In the outboard engine unit disclosed in JP-A-2010-221754, an output shaft of a transmission undergoes forward rotation during forward propulsion of a hull and reverse rotation during reverse propulsion of the hull. The output shaft of the transmission stops its rotation during stop of the hull. Where the water pump is disposed on the output shaft of the transmission, the water pump cannot pump out a cooling water to an engine for cooling the engine during the reverse propulsion or stop of the hull.
To address this problem, it is suggested that the water pump be disposed separately from the output shaft of the transmission, that is, the water pump be disconnected from the output shaft, in which case the water pump may be connected through a drive mechanism to an input shaft of the transmission. Since the input shaft of the transmission continues forward rotation, the water pump can be driven to feed a cooling water to the engine even during reverse propulsion or stop of the hull, as in the case of forward propulsion of the hull. However, where the water pump is disposed separately from the output shaft of the transmission, a part for attaching the water pump separately from the output shaft of the transmission or a drive mechanism for connecting the water pump to the input shaft of the transmission is required. As a result, the number of parts of the outboard engine unit would increase and hence the weight of the outboard engine unit would increase.
In the outboard engine unit, the transmission transmits rotation of an engine crankshaft to a propeller shaft in such a manner as to adjust propulsion of the hull.
Since the outboard engine unit includes the transmission, however, the outboard engine unit is required to have not only an engine-cooling system but also a transmission-cooling system separate from the engine-cooling system. The two systems, that is, the engine-cooling system and the transmission-cooling system in the outboard engine unit make a structure of the engine-cooling system complicated and hence cost reduction of the outboard engine unit difficult.